In the past year, our efforts focus on the role of SIRT1 in hepatic and intestinal nutrient metabolism and tissue homeostasis, as well as mechanisms and physiological importance of SIRT1 Thr522 phosphorylation. As a highly conserved NAD+-dependent protein deacetylase, SIRT1 has been shown as a key metabolic sensor that directly links nutrient signals to animal metabolic homeostasis. Although the functions of SIRT1 have been extensively studied in various metabolic tissues in recent years, the role of SIRT1 in nutrient absorption and sensing in small intestine, a key metabolic organ that provides the first interface between nutrients and animal metabolism, is still completely unknown. To elucidate the function of SIRT1 in intestinal metabolism, we recently generated a novel intestine-specific SIRT1 KO mouse model, SIRT1 IKO mice. SIRT1 IKO mice were morphologically normal under standard feeding conditions, however, they displayed significantly lower serum and hepatic bile acid levels than control animals, suggesting a defect in bile acid metabolism. Further analyses demonstrate that intestinal SIRT1 is an important regulator of ileal bile acid absorption that feedback modulates systemic bile acid homeostasis and cholesterol metabolism. Specific deletion of SIRT1 in intestine decreases the intestinal HNF1&#945;/FXR signaling pathway, thereby reducing expression of the bile acid transporter genes Asbt and Ost&#945;/Ost&#946;, and absorption of ileal bile acids. We provide evidence that SIRT1 regulates the HNF1&#945;/FXR signaling pathway partially through DCoH2, a dimerization cofactor of HNF1&#945;. SIRT1 deacetylates DCoH2, facilitating dimerization and DNA binding of HNF1&#945;. Furthermore, intestinal SIRT1 deficiency decreases the expression of FGF15, which in turn alleviates the inhibition of hepatic bile acid synthesis, reducing hepatic bile acid levels and decreasing liver damage upon high bile acid diets feeding. Our findings uncovered a novel function of SIRT1 in intestinal bile acid absorption and revealed a previously unknown molecular mechanism by which SIRT1 regulates the HNF1&#945;/FXR pathway. Our studies further point out that the same molecular mechanism can yield distinct pathophysiologies in the same metabolic pathway in different tissues, and suggest that tissue specificity should be considered when applying SIRT1 small molecule modulators-based therapeutic strategies to bile acid and cholesterol diseases. Currently, a manuscript describing this study is ready for submission. The small intestine is not only essential for nutrient absorption and sensing, but also provides the first line of defense against pathogenic microbes as well as various environmental agents such as diet, drugs, and toxins. As a result, the intestinal epithelium is under rapid renewal every 3-5 days. Therefore, intestine is a perfect system for studies on interplays between nutrient metabolism, host defense, and tissue homeostasis. Through a microarray analysis of mRNA from the ileum and duodenum of control and SIRT1 IKO mice, we discovered that deletion of intestinal SIRT1 significantly increased the expression of Paneth cell markers, including antimicrobial peptides defensins, lysozyme, and lectin Reg3b, proteins in the Wnt signaling pathway, as well as metallothionein 1 (MT1), a protein that is expressed predominately, although not exclusively, in Paneth cells. Based on our preliminary data, we hypothesize that intestinal SIRT1 is a key regulator of Paneth cell differentiation and gene expression, playing an important role in Paneth cell related physiologies, including intestinal microbiota formation and inflammation. Currently we are working to test our hypothesis. While much attention has been focused on the identification of cellular targets to explore the molecular mechanisms and functional networks controlled by SIRT1, how SIRT1 activity is regulated is still unclear. Therefore, another focus of our group is to understand the molecular mechanism that controls SIRT1 expression and activity. In our previous studies, we reported that SIRT1 is activated by phosphorylation at a conserved Thr522 residue in reesponse to environmental stress, and this modification activates SIRT1 through modulation of its oligomeric status. To test whether phosphorylation of Thr522 of SIRT1 is an important post-translational modification that modulates physiological SIRT1 activity in vivo, we recently generated two knock-in mouse models, SIRT1KIB6TA and SIRT1KIB6TE, with T522A (TA) to mimic the dephosphorylated status and T522E (TE) to mimic the phosphorylated status of SIRT1. Both KI strains were phenotypically normal under standard feeding conditions. However, SIRT1 TE mice had enhanced ability to mobilize lipids from white adipose tissue and displayed increased fatty acid &#946;-oxidation activity in liver in response to fasting. SIRT1 TA mice, on the other hand, accumulated significantly a higher amount of triglycerides and free fatty acids (NEFA) in liver compared to WT and TE mice after 16 weeks of western style high-fat diet feeding. Currently we are working to repeat and verify these observations with a larger cohort of mice within different age groups. We will then analyze the expression of genes involved in glucose and lipid metabolism, as well as other related phenotypes in energy metabolism.